1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus for heating a thin plate-like precision electronic substrate (hereinafter referred to simply as a “substrate”) having a back surface of a known emissivity such as a semiconductor wafer and a glass substrate for a liquid crystal display device by irradiating the substrate with light.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short period of time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short period of time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short period of time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short period of time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
U.S. Pat. No. 7,935,913 discloses the technique in which a light measuring part including a calorimeter disposed outside a chamber body, a light guide structure for guiding the light emitted to the inside of the chamber body to the calorimeter, and a calculation part that performs computations based on an output from the calorimeter is provided in a flash lamp annealer, to thereby measure the energy of the light emitted to the inside of the chamber body from a flash lamp with the calorimeter. In addition, U.S. Pat. No. 7,935,913 discloses that the surface temperature of a substrate is obtained by computations based on the energy of a flash of light measured by the calorimeter.
In the technique disclosed in U.S. Pat. No. 7,935,913, the total energy (amount of heat) of single flash irradiation is measured, to thereby obtain the maximum attained temperature of the surface of the substrate from the total energy. However, even if the total energy of flash irradiation is constant, the light energy to be absorbed differs between different emissivities of surfaces of a semiconductor wafer, which causes variations in the surface temperature to be attained. Typically, the flash lamp annealing is performed for activating impurities implanted into a source and a drain formed in the surface of a semiconductor wafer. In other words, fine device patterns including source and drain regions are formed in the surface of the semiconductor wafer on which flash lamp annealing is performed, and an emissivity frequently differs depending on patterns. This makes it extremely difficult to measure the surface temperature of the semiconductor wafer to be actually treated or processed in which impurities have been implanted.